center pontoon on houseboat

I purchased a 28' pontoon boat to convert into a trailerable houseboat. The pontoon is light duty and building a trailerable cabin at the current displacement weight is impossible. I plan a center pontoon with a flat bottom and straight sides at a shallower draft than the side pontoons so it will float at rest and plane at speed. The center pontoon will be plywood (foam filled) and fiberglassed and run from the fore end to a "false transom scoop" of aluminum already on the boat. My question is- what transom angle on the center pontoon and what distance from the center pontoon to the aluminum "false transom" will leave the outboard in "clean" water?

Thanks